The present invention relates generally to load cell sensing devices such as utilized in connection with apparatus for indicating the loads sustained by the lines, cable and the like in cranes, derricks, and similar equipment.
Heretofore, load cell sensing devices have in the main comprised two general types which have been commonly referred to as a hydraulic load cell and an electronic or electric load cell.
Hydraulic load cells have been used for a number of years and can briefly be described as instruments which measure force or load information by means of hydraulic pressure and are usually connected to an indicator such as a Bourdon tube for translation of hydraulic pressure into force or weight indications on an indicating or recording instrument. The hydraulic load cell is of rugged dependable construction and is susceptible of repair and overhaul in the field by the usual technician. The hydraulic load cell poses one disadvantage, however, in that the distance at which a remote indicator may be positioned is limited due to line resistance and response lag.
More recently, consideration has been given to the possibility of utilizing electronic or electric load cells which have the advantage of permitting the establishment of the indicating instrumentation at remotely distant positions which are not possible in the case of the hydraulic load cell. However, the electronic or electric load cell is rather delicate and sensitive to such an extent that these cells in certain installations are less dependable than the hydraulic load cell. Also, the electronic or electric load cell requires more specialized personnel for repair and maintenance.
While the usual load indicating apparatus has in most cases utilized one or the other of these load cells as the sensing medium, there are heretofore known arrangements in which several load sensing cells have been utilized in combination. For example, there is disclosed in U.S. Pat. No. 3,229,515, issued Jan. 18, 1966, an arrangement wherein a hydraulic load cell is utilized as the primary sensing device, and an electric load cell in the form of a transducer is utilized for converting the variations of the hydraulic fluid in the hydraulic load cell into electric signals.
In U.S. Pat. No. 3,295,086, issued Dec. 27, 1966, a plurality of load cells of the electronic or electric type with different load ranges are mechanically connected in series to provide a multi-range load sensing cell assembly.
Also, from U.S. Pat. No. 3,472,069, issued Oct. 14, 1969, it has been known to provide a load cell protecting device in which a by-pass mechanism is utilized to temporarily remove the load forces from the load cell between successive applications.
The present invention seeks to provide a load sensing arrangement which combines an electronic or electric type load cell and hydraulic load cell in such a manner that they may be operated selectively to sense the load, and wherein the load cells mutually provide a back-up for each other.
During the early stages of development of the general concept of the present invention, it was thought that the hydraulic load cell and the electronic or electric load cell could be combined and utilized respectively for load sensing, simply by mounting them mechanically in stacked relation, each of the load cells being individually connected to its particular indicator. It soon became apparent, however, that the desired results could not be obtained in this manner because of the different response characteristics of the two types of load cells. The electronic or electric load cell, by virtue of its being very sensitive and operable upon relatively small load displacements of the order of ten-thousandths of an inch in a load range of, e.g., 0-40,000 pounds, is incompatible with the larger load displacement movements in the hydraulic cell wherein the movements are in fractions of an inch. It thus became evident that with the two cells in stacked relation, the desired results would not be obtained, since the sensitive electronic cell would operate only to indicate the sensitive pulsations and fluctuations in the hydraulic cell, and would not therefore provide a true load indication.
The foregoing problem has been solved in the present invention by providing unique means for selectively by-passing the load with respect to each cell. This unique concept thus makes it possible to combine the more sensitive electronic cell with the more rugged hydraulic cell in a manner such that the advantageous features of both cells may be utilized. With such an arrangement, the electronic load cell would be normally utilized to indicate the load forces, while the dependable and rugged hydraulic cell would be maintained in an inactive condition by by-passing the load forces around it, and so as to serve as a readily available back-up which could be put into active service in place of the electronic cell, at any time at a minimum cost.